This invention relates to an electromagnetic assembly comprising an electrical coil having a magnetic core which is of the split pole type in which one of at least two poles is surrounded by a shading ring. An armature arranged opposite to the poles and has a yoke which closes the magnetic circuit of said assembly.
Such electromagnetic assemblies are used in many fields of technology, for example, in state-of-the-art relays driven by means of alternating current. Dividing the core opposite to the armature into two or more poles, one of which is surrounded by a shading ring, serves to produce a phase shift in the magnetic flux in the region of the two poles, when the electrical coil is energized. Said phase shift prevents interruption of the magnetic retaining force between core and armature when the alternating current changes direction. Such electromagnetic assemblies are described for example in German Patent Specifications 141 026B1 and 539 918B1.
In order to produce as great a possible magnetic force between the poles and the armature, in split-pole cores of laminated construction, the area surrounded by the shading ring should be as large as possible, so that, the magnetic resistance via that flux path is kept low. By virtue of this expedient favourable flux conditions and also favourable output conditions with respect to the phase shift between the two partial fluxes can be obtained. The core must, however, be laminated in order to achieve homogeneous magnetic field distribution. Effective magnetic field displacement will not occur if the laminations are of suitable thickness since the eddy currents in the core laminations will be very small.
For manufacturing reasons alone, however, it is desirable to use in alternating current relays, for example, unlaminated cores when the cores exceed a given size. Magnetic field displacement caused by eddy currents induced in the unlaminated core, results in low depth of penetration of the magnetic field, with associated non-homogeneous magnetic field conditions in the immediate environment of the poles. Such conditions adversely affect the magnetic forces exerted between the poles and the armature, since the flux densities over the pole surfaces thus also depend on location and vary.
As the pole surface area surrounded by the shading ring is relatively large, the effect of said field displacement is substantial.